most of the material appears to be not Alter Rebbe
As for the link above. I can’t open it with my filter and there is no siman 148 in the SA Harav (what is the summary?)
The link is a bit misleading, I should have clarified:
Shulchanaruchharav. com is a website with thousands of halacha articles on various subjects, not necessarily based on Shulchan Aruch Harav.
This is what it says there (there are also lengthy references in the footnotes which I didn’t bring):
May one wish a merry Chris-mas, or happy holidays, to a gentile acquaintance or neighbor?
Merry chris-mass:[6] The term Chris-mas is not to be mentioned, due to the prohibition against mentioning the name of idols.
Other greetings: If the gentile does not believe in the religious connotations behind the holiday, then there is no prohibition to mention to him “Happy Holidays.”[7] If, however, the gentile believes in the idolatry related content behind Christmas, seemingly, one may only do so in a pressing situation, to avoid enmity.[8] Certainly one should not go out of one’s way to greet him and send him Holiday wishes, such as through social media, unless lack of doing so will cause enmity. In all cases that one meets a gentile acquaintance outside who is a practicing Christian, he is to greet him with a low voice, as stated above.
May one wish others a happy New Years on the 1st of January?[9]
There is no prohibition involved in wishing a happy New Years to a gentile who does not affiliate the day with any Christian connotations or worship of a deity.[10] One is to avoid wishing a happy New Years to a practicing Christian [particularly Catholics, and Lutherans] who believes in the Christian doctrine behind the New Year’s Holiday.[11] However, even in such a case, one may do so in a pressing situation, in order to avoid causing enmity and anti-Semitism.[12] Certainly one should not go out of one’s way to greet him and send him Holiday wishes, such as through social media, unless lack of doing so will cause enmity. In all cases that one meets a gentile acquaintance outside who is a practicing Christian and believes in the holiday of New Years, he is to greet him with a low voice, as stated above. [In general, it is not customary of Jews to wish other Jews a Happy new year on the first of January.[13] However, it is related, that Rebbe Levi Yitzchak of Berditchiv would wish others a Happy new year on the 1st of January. Likewise, the Rebbe once wished a Chassid a happy new years on the morning of January first, in continuation of the tradition from Rebbe Levi Yitzchak.[14] This is based on the verse in psalms 87:6 ““Hashem Yispor Bichsov Amim..”]